The invention relates to a roll coating and in particular to a coating which is suitable for rolls used in the production process of a paper web, board web, tissue web, pulp web or other fiber web, and to a method for the manufacture of such coating. The invention also relates to a roll of a fiber web machine, where the roll comprises a coating of a polymeric material on the roll body.
Roll coatings commonly used in the production of paper, board and pulp require a wide range of technical features, such as good dynamic endurance under high load, wear resistance, and properties required by the process or by the fiber web, such as a suitable hardness. Such coatings are typically manufactured from polymers such as polyurethane elastomer. The polyurethane elastomer is typically made from a urethane prepolymer and a chain extender, or a hardening agent, which makes the prepolymer chain become longer by combining long prepolymer chains together, whereby the product becomes viscous, in other words it hardens.
Polyurethane grades based on aromatic isocyanates and especially ones based on PPDI (para-phenylene diisocyanate) are considered as the best raw material basis of present polyurethane roll coatings. They provide excellent heat-resistance and water-resistance and low heat generation, which correlates with the load endurance and cooling need of the coating. Like other polyurethane coatings, PPDI coatings do not usually need cooling, which results in significant energy savings. However, a drawback of the hardest PPDI coatings, with a hardness below 12 P&J (Pusey & Jones) or correspondingly above 96 Shore A, is their poor strength while they are fresh in the manufacturing stage at the moment when the material begins to harden and shrink around the roll. The brittle stage typically lasts for approximately 15 to 60 minutes, after which the chemical hardening reaction has progressed sufficiently far and the material is strong. However, it has been found that the shrinkage of hard PPDI coatings during the brittle stage causes cracking of the coating when the coating is cast around the roll.
FI utility model FI 8255 describes a roll coating which comprises at least one polyurethane elastomer, at least one filler and fibers, and potentially a cord fabric. The publication presents that such a coating has good compression and wear resistance, tearing strength, hardness, and impact strength. According to the publication, the polyurethane preferably contains constituents which include alifatic and/or cycloalifatic isocyanate groups, because these constituents allow an extension to the time between the mixing of the polyurethane pre-stage compounds and the hardening of the mixture. The roll coating can be manufactured by impregnating the cord fabric in the mixture of the polyurethane reaction components or by applying the mixture through a nozzle onto the support fabric, after which the support fabric is wound around the roll.
WO publications 2007/128596 A1 and 2009/033874 A1 describe a roll coating which comprises PPDI-based polyurethane. According to the publication, the strength of the polyurethane while it is fresh is improved and its stress cracking is reduced by adding to the polyurethane a hardening agent which contains 60-99 percent by weight of 1,4-butanediol, a maximum of 40 percent of diamine, and a maximum of 1 percent of catalyst. The hardness of the polyurethane is approx. 80 Shore A-75 Shore D. In the publication, the roll coating is manufactured by the strip casting technique.
A problem in prior art roll coatings is their tendency to generate heat due to the dynamic deformation caused by the roll nip. In most cases, the roll coating being subject to a high load taking place several times per second, the heat generation results in the melting or thermal destruction of the coating if the coating is not cooled from the inside of the roll. Cooling requires suitable equipment to be arranged on the roll and appropriate monitoring of this equipment, which cause extra work. Another disadvantage is that cooling increases the energy consumption of the fiber web.
The very low heat generation and good mechanical properties of roll coatings manufactured from PPDI-based polyurethanes are undisputed. The coatings of press and suction rolls used in paper, board and pulp production have a distinct need for PPDI-based polyurethane roll coatings which have a hardness of below 12 P&J and which can be manufactured in a simple, efficient and economical manner.